A hydraulic construction machine in accordance with an embodiment of the prior art as shown in FIG. 1 includes:
a first variable displacement hydraulic pump (hereinafter, referred to as “first hydraulic pump”) 1 and a second variable displacement hydraulic pump (hereinafter, referred to as “second hydraulic pump”) 2, which are operated by an engine (not shown) or the like;
a traveling motor 3 and a swing motor 4, which are operated by hydraulic fluid supplied from the first hydraulic pump 1;
hydraulic actuators 5, 6 and 7, which are operated by hydraulic fluid supplied from the second hydraulic pump 2; and
a main control valve (MCV) 15 including first control valves 9 and 10 that are installed in a supply flow path 8 of the first hydraulic pump 1 and is shifted to control a flow direction of the hydraulic fluid supplied from the first hydraulic pump 1 to the traveling motor 3 and the swing motor 4, and second control valves 12, 13 and 14 that are installed in a supply flow path 11 of the second hydraulic pump 2 and is shifted to control a flow direction of the hydraulic fluid supplied from the second hydraulic pump 2 to the hydraulic actuators 5, 6 and 7.
According to the construction as described above, when a manipulation lever (i.e., RCV lever) is manipulated in order to swing the machine, a spool of the first control valve 10 is shifted to the left or right on the drawing sheet in response to a control signal according to the manipulation of the manipulation lever so that hydraulic fluid discharged from the first hydraulic pump 1 is supplied to the swing motor 4 via the spool of the first control valve 10 along a supply flow path 8. Thus, the machine can be operated to swing separately.
When a travel pedal is manipulated in order to travel the machine, a spool of the first control valve 9 is shifted to the left or right on the drawing sheet in response to a control signal according to the manipulation of the travel pedal so that the hydraulic fluid discharged from the first hydraulic pump 1 is supplied to the traveling motor 3 via the spool of the first control valve 9 along the supply flow path 8. Thus, the machine can be operated to travel separately. As described above, when the machine is caused to swing or travel separately, the hydraulic fluid supplied at a maximum discharge flow rate from the first hydraulic pump 1 can be used
In the meantime, when a combined operation is performed in which the traveling apparatus and the swing apparatus are operated simultaneously, a maximum discharge flow rate of the hydraulic fluid is dispensively supplied from the first hydraulic pump 1 to the traveling motor 3 and the swing motor 4. In this case, there is caused a problem in that the work efficiency is deteriorated due to a reduction in the operating speed of the traveling motor 3 and the swing motor 4, and a shock is delivered to an operator who is operating the machine due to a sudden reduction in the traveling speed and the swing speed during the traveling or swing operation of the machine, thus leading to an increase in the operator's fatigue and a decrease in the manipulability.
A hydraulic construction machine in accordance with another embodiment of the prior art as shown in FIG. 2 includes:
a first variable displacement hydraulic pump (hereinafter, referred to as “first hydraulic pump”) 1 and a second variable displacement hydraulic pump (hereinafter, referred to as “second hydraulic pump”) 2, which are operated by an engine (not shown) or the like;
a traveling motor 3 and a swing motor 4, which are operated by hydraulic fluid supplied from the first hydraulic pump 1;
hydraulic actuators 5, 6 and 7, which are operated by hydraulic fluid supplied from the second hydraulic pump 2;
a main control valve (MCV) 15 including first control valves 9 and 10 that are installed in a supply flow path 8 of the first hydraulic pump 1 and is shifted to control a flow direction of the hydraulic fluid supplied from the first hydraulic pump 1 to the traveling motor 3 and the swing motor 4, and second control valves 12, 13 and 14 that are installed in a supply flow path 11 of the second hydraulic pump 2 and is shifted to control a flow direction of the hydraulic fluid supplied from the second hydraulic pump 2 to the hydraulic actuators 5, 6 and 7;
a selection valve 16 that is installed on the most upstream side of the supply flow path 11 of the second hydraulic pump 2 and is shifted to supply the hydraulic fluid from the first hydraulic pump 1 to the traveling motor 3 and the hydraulic actuators 5, 6 and 7 and to supply the hydraulic fluid from the second hydraulic pump 2 to the swing motor 4 and the hydraulic actuators 5, 6 and 7 in response to a control signal that is applied thereto when the traveling motor 3 and the swing motor 4 are simultaneously operated; and
a control valve 17 that is installed on the most downstream side of the supply flow path 11 of the second hydraulic pump 2 and is shifted to close an opening of the most downstream side of the supply flow path 11 of the second hydraulic pump 2 in response to the control signal that is applied thereto when the traveling motor 3 and the swing motor 4 are simultaneously operated.
According to the construction as described above, when the traveling motor 3 and the swing motor 4 are simultaneously operated, the shift of the selection valve 16 supplies the hydraulic fluid from the first hydraulic pump 1 to the traveling motor 3 and the hydraulic actuators 5, 6 and 7, and supplies the hydraulic fluid from the second hydraulic pump 2 to the swing motor 4 and the hydraulic actuators 5, 6 and 7.
In this case, the hydraulic fluid from the first hydraulic pump 1 is supplied to a side whose pressure is relatively low due to a difference in the load pressure between the traveling motor 3 and the hydraulic actuators 5, 6 and 7, and the hydraulic fluid from the first hydraulic pump 1 is supplied to a side whose pressure is relatively low due to a difference in the load pressure between the swing motor 4 and the hydraulic actuators 5, 6 and 7.
For this reason, a flow rate of the hydraulic fluid respectively supplied from the first hydraulic pump 1 and the second hydraulic pump 2 to the traveling motor 3 and the swing motor 4 is variable due to the load pressure generated between the traveling motor 3 and the hydraulic actuators 5, 6 and 7 and the load pressure generated between the swing motor 4 and the hydraulic actuators 5, 6 and 7, thus causing a problem in that the manipulability of the traveling apparatus and the swing apparatus is decreased.